Does heat soften rubber mousepads and affect performance?

Yes, technically, but the effect is small unless you’re in extreme conditions. The rubber compound is designed to stay stable across normal indoor temperatures, so the difference between 25°C and 35°C is basically nonexistent for most people. Humidity is a bigger factor—moisture can make the cloth surface tacky and slow glide. Unless you’re a competitive FPS player, don’t overthink it.